The invention relates to a method of determining the exposure duration for a photographic copying material having different gradations for light of different colors, when the exposure of such copying material is performed using light of two colors. More particularly, the invention relates to forming from an original having a certain density range a copy having a desired different density range, by proper selection of the exposure times in the two colors of exposure light. Also, the invention relates to apparatuses for performing the process.
With a known process of the general type in question, gray wedge images are made for different ratios of the exposures in the two different colors, and isodensity lines are determined for the desired values of the copy density. These isodensity lines are graphically determined and constitute a basis for the subsequent determination of the relationship between the two exposures in the two different colors and the total exposure time, for copying a particular original exhibiting a range of densities which has been determined previously. This graphical method of determination requires considerable expense for the formation of the requisite gray wedge images; likewise, the evaluation of the gray wedge images for the purpose of plotting the isodensity lines requires considerable time.